Find the time constant for the given circuit.

$A$ $20 \mu F$ capacitor is connected to a $45 \text{ V}$ battery through a circuit whose resistance is $2000 \Omega$. What is the final charge on the capacitor?

$A$ charged capacitor is allowed to discharge through a resistor by closing the key at the instant $t = 0$. At the instant $t = (\ln 4) \, \mu s$,the reading of the ammeter falls to half the initial value. The resistance of the ammeter is equal to

$A$ capacitor is discharging through a resistor $R$. Consider in time $t_{1}$,the energy stored in the capacitor reduces to half of its initial value and in time $t_{2}$,the charge stored reduces to one-eighth of its initial value. The ratio $t_{1} / t_{2}$ will be ................

$A$ light bulb,a capacitor,and a battery are connected in a series circuit as shown in the figure,with switch $S$ initially open. When the switch $S$ is closed,which one of the following is true?

$A$ $30\,\mu F$ capacitor is charged by a constant current of $30\, mA$. If the capacitor is initially uncharged,how long does it take for the potential difference to reach $400\, V$?